could not find implicit value for parameter tokensReader: mainargs.TokensReader[os.Path]

[james-s-w-clark]

Continuation from Discord thread.

I started from the simple reproducible case (thanks @Baccata for raising) and tried some of the suggested changes in my mill fork.

In summary I understand there are two concerns:

1. It would be nice to have implicit value for parameter tokensReader: mainargs.TokensReader[os.Path] available by default
2. Custom TokensReaders aren't discovered. The mainargs library provides a few readers (String, Boolean, Byte, etc.)

The former works with adding import mill.runner.MillCliConfigParser.PathRead to MillBuildRootModule, but I didn't have luck with the latter.

I didn't raise a PR here with the changes as I suppose we don't want a custom TokensReader example (covered in mainargs readme).